The use of color in the digital environment presents continual problems with regard to accuracy and matching of colors that are intended to appear the same when presented through different devices or on different mediums. Specifically, it is hoped that a color can be perceived as the same even though viewed on a photograph and then scanned into the digital environment, displayed on a CRT monitor or printed on a color printer. Since each of these elements involve a different form of color definition in a distinct color space, the transformation of color through the different color spaces while maintaining a perceptually accurate matching, is a difficult problem to solve. More particularly, how a color appears on a color photograph is typically defined by one form of colorimetric data, i.e., a device independent color space, such as CIE XYZ or CIE Lab (for general background discussion of different color spaces see Billmeyer and Saltzman, Principles of Color Technology, 2nd Ed., Wiley and Sons, 1981; Russ, The Image Processing Handbook. 2nd Ed., CRC Press, 1995 ). The data signals for colors for a printer are defined in a device dependent color space, such as CMY or CMYK. Most calibration of a transformation between a device dependent color space and a device independent color space occurs through a lookup table (LUT) comprised of a limited number of predetermined matches between a standard target and the device output. For example, a common industry target is a Kodak Q60 target comprised of 264 different color patches which can be used to calibrate a device such as a printer for those limited 264 colors. For the literally millions of colors that could be printed by a printer in addition to the exemplary 264 patches from the target, some form of interpolation about the predetermined values is employed.
For detailed discussions of color printing and calibrating systems, the following commonly-assigned patents should be referenced:
U.S. Pat. No. 5,787,193 U.S. Pat. No. 5,528,386 U.S. Pat. No. 5,739,927 U.S. Pat. No. 5,483,360 U.S. Pat. No. 5,689,350 U.S. Pat. No. 5,471,324 U.S. Pat. No. 5,649,072 U.S. Pat. No. 5,416,613 U.S. Pat. No. 5,594,557 U.S. Pat. No. 5,307,182 U.S. Pat. No. 5,592,591 U.S. Pat. No. 5,305,119 U.S. Pat. No. 5,581,376
All of which patents are herein incorporated by reference.
There are many ways to implement a calibrated lookup table in a digital signal processing system. The present invention is applicable to any method implemented either by software-based or hardware-based algorithms. A demonstration of a particular application, which is described later for the present invention, is a software implemented algorithm for effecting the lookup table by a three-layer feedforward neural network. Such neural networks are common and well known and can be referenced in Timothy Masters, Practical Neural Networks in C++, Academic Press, Inc. 1993.
The transformation process by which a colorimetric data signal is converted from one color space to another is conventionally referred to as a transform. For printer calibration, the transformation from a device dependent color space, i.e., CMYK, to a device independent color space, i.e., CIE Lab, is often referred to as a forward transform [T(CMYK).fwdarw.Lab], while the transformation from the device independent color space to the device dependent color space is referred to an inverse transform [T.sup.-1 (Lab).fwdarw.CMYK].
It is a fact of the transforming process that the forward transform, T(CMYK).fwdarw.Lab, provides a more accurate color match than an inverse form of the transform, T.sup.-1 (Lab).fwdarw.CMYK.
Keeping in mind that the printer calibration of the transform processing is merely a mathematical modeling, reasons for the disparity in accuracy between forward and inverse transforms can be better appreciated. First, the respective color spaces can be of different dimensions, such as when transforming from a three-dimensional space, Lab, to a four-dimensional space, CMYK. Interpolation relative to the fourth dimension for calibrating a color match presents a particular problem for accuracy. Secondly, there is no problem with the forward transform that the transformed data signal will be out-of-gamut, because the input color space is device dependent and the color gamut is defined by the limit of colors which could be generated by the corresponding printer. On the other hand, the input color space of an inverse transform is device independent and the output colors could be outside the gamut defined by the calibrated printer. Another reason for higher accuracy for the forward transform is that the mathematical modeling is evaluated in the Lab space, which is defined as a uniform color space in terms of human visual perception, and usually yields better optimization results.
The present invention takes advantage of the disparity in accuracy between the forward and inverse forms of the transform to calibrate or adjust an input color data signal during the transformation process itself to compensate for the detected inaccuracy of the inverse transform and thereby provide improved results.